Chap. 10] 



ELECTRICAL METHODS 



651 



read on the simulating unit, the resistivity and dielectric constants can be 

 calculated as in the resonance method (see paragraph 1). 



3. Phase-shift method. The phase shift between the resultant current 

 and the conductance component can be determined directly, in the fol- 

 lowing manner (Fig. 10-17) : A perfect air condenser Cp without appreci- 

 able loss angle is connected in series with the soil condenser Cs . Both 

 are placed in a high-frequency circuit and the voltages across the soil 

 condenser and across the perfect condenser are- compared on a cathode 

 ray oscilloscope. The current through the soil condenser Ir makes the 

 phase angle <p with the pure conductance component. If a perfect con- 

 denser is now placed in series with the soil condenser, the e.m.f. across 

 it will be 90° out of phase with respect to this current and, therefore, have 

 a phase shift of 90 — y? compared with the e.m.f. across the condenser. 



Tuning 



# 



RF Dtt 



Simulating 

 Unit 



'///////, 



i.ij.tttj. 



Beat 

 Osclifator "TA 



r ilAms/ifJtr 



V//////M 



XtSTS 



^hI 



Fig. 10-16. Determination of reactive and conductive components of soils at radio 

 frequencies (adapted from Feldman). 



If the condenser terminals are then connected to the four plates of a 

 cathode-ray oscilloscope as shown in Fig. 10-17c, the light spot will trace 

 the resultant of the two voltage components and thus give their phase 

 shift directly.^" 



4. Polarization measurements. In this connection, the term "polariza- 

 tion" refers to the polarization of radio waves in the course of their propa- 

 gation over the earth. This is brought about in the following manner: 

 The radiation from an antenna may be considered as consisting of three 

 component parts: (a) a space wave, whose amplitude decreases inversely 

 ivith the distance from the source because of a geometric spreading; (6) a 

 surface wave, decreasing in amplitude inversely with the square root of 

 distance] and (c) a ground wave of a depth penetration generally smaller 



1" C. B. Feldman, Bell Lab. Rec, 12(12) (Aug., 1934). Ratcliffe and White, loc. 

 cit. R. L. Smith-Rose, Roy. Soc. Proc, A130, 359 (1933). 



